Question

It takes 26.8 mL of a 0.0700 M NaOH standard solution to neutralize a 250 mL sample of lactic acid (C3H6O3). What mass of lactic acid was dissolved in the sample?

Answer 1

There is 1 OH- in 1 molecule of NaOH. 
Also, there is 1 H+ in 1 molecule of lactic acid.

So the reaction is simple.
so just equate the moles
moles of OH- in NaOH = moles of H+ in lactic acid
26.8 x 0.07 = 250 x Mole of lactic
Moles of lactic = 0.0075

so mass = 0.0075 x 90.8 =  0.681 g